1. Field of the Invention
The present invention relates to a sustained release tablet designed or adapted to provide an immediate release of an active agent, a period of no release of an active agent, followed by a substantially constant release of the active agent.
2. Description of the Prior Art
Phenylpropanolamine hydrochloride .alpha.-(aminoethyl)benzyl alcohol hydrochloride, also known as dl-norephedrine hydrochloride, 2-amino-1-phenyl-1-propanol hydrochloride, and .alpha.-hydroxy-.beta.-aminopropylbenzene hydrochloride), is a well-known sympathomimetic amine. Phenylpropanolamine hydrochloride is well documented as a therapeutic agent which is used as an anorexiant for control of obesity. In this capacity it has been marketed by, among others, SDA Pharmaceuticals, Inc. under the trademarks Anorexin.RTM. and One-Span.RTM., and by Thompson Medical Company, Inc. under the trademark Dexatrim.RTM..
Phenylpropanolamine hydrochloride is also a bronchial dilator, and is accordingly used for the treatment of asthma, as well as being commonly employed as a decongestant for treatment of upper respiratory tract congestion. As an antiasthmatic, phenylpropanolamine hydrochloride has been marketed by Eaton Laboratories under the trademark Rymed.RTM..
U.S. Pat. No. 4,971,798 describes a slow-dissolving lozenge confection to provide slow sustained release of an antitussive, decongestant, antihistamine, or expectorant ingredient, which may be phenylpropanolamine.
U.S. Pat. No. 4,894,223 describes a novel drug delivery system for decongestants. The delivery system, which may be used to deliver phenylpropanolamine hydrochloride, is in dry particulate form, and includes a hydrophobic matrix and a coat.
It is an object of the present invention to provide a sustained release form of an active ingredient, such as Phenylpropanolamine hydrochloride, which is characterized by immediate release of the active ingredient, a period of no release of active agent, followed by release of the active ingredient at a uniform constant rate, independent of the drug concentration and/or gastrointestinal pH variation during the period of release